1. Field of the Invention
The invention relates to a sheet processing apparatus which folds a sheet bundle, and particularly almost prevents the open ends of a folded sheet bundle from opening, and an image forming apparatus provided with this sheet processing apparatus in a main body of the image forming apparatus.
2. Related Background Art
There is, for example, a sheet processing apparatus which binds sheets into a bundle shape. This sheet processing apparatus is adapted to be capable of performing the processing of making sheets into a bundle shape and needle-binding the bundle at its intermediate portion, and then folding it into the shape of a pamphlet (so-called booklet (saddle-stitched book) making, i.e., saddle stitching). Such a sheet processing apparatus is sometimes provided as a constituent of an image forming apparatus in the apparatus main body of the image forming apparatus (see Japanese Patent Application Laid-Open No. 2000-153947).
FIG. 14 of the accompanying drawings shows the image forming apparatus provided with that sheet processing apparatus. The conventional sheet processing apparatus 901 shown in FIG. 14 is adapted to be capable of performing the above-mentioned saddle stitching, the aligning process of aligning the end portion of a sheet bundle (so-called sorting process), the side stitching of binding the end portion of the sheet bundle (so-called staple sorting process), etc.
The conventional sheet processing apparatus 901 performs the saddle stitching in the following manner. The sheet processing apparatus 901 stacks sheets successively received from the apparatus main body 903 of the image forming apparatus 902 onto a saddle stitching tray 910 formed steeply and substantially straight, receives the sheets by a stopper 911 and makes them into a bundle shape. Then, a width aligning device (not shown) performs the width aligning of the sheets P. Thereafter, an intermediate portion stapler unit 912 binds the sheet bundle at two locations near the substantially central portion thereof. The stopper 911 is moved up and down to thereby oppose the bound portions of the bound sheet bundle to the nip between a pair of folding rollers 914 and a thrusting plate 913. The thrusting plate 913 thrusts the bound portions of the sheet bundle and sends the sheet bundle into the nip between the pair of folding rollers 914. The pair of folding rollers 914 fold the sheet bundle into two while nipping and conveying the sheet bundle. Lastly, a pair of discharging rollers 915 discharge the sheet bundle onto a sheet stacking portion 905. As described above, the conventional sheet processing apparatus 901 can make the sheet bundle into the shape of a twice-folded pamphlet.
Also, the sheet processing apparatus 901 performs the staple sorting process in the following manner. The sheet processing apparatus 901 successively receives the sheets P having had images formed on one side or two sides thereof in the apparatus main body 903 of the image forming apparatus 902 by an intermediate tray 906 and at the same time, aligns the width (width-aligns) of the sheets by a width aligning device 907 and makes the sheets into a bundle shape. Thereafter, an end portion stapler unit 908 binds the end portion of the sheet bundle. Lastly, a pair of discharging rollers 909 discharge the sheet bundle onto a sheet stacking portion 904.
In the conventional sheet processing apparatus, however, when the booklet is made, if folding pressure is weak or the sheets are sheets of a material difficult to fold, it has sometimes happened that the open ends of the sheet bundle in the shape of a twice-folded pamphlet somewhat may be opened and the dignity of the sheet bundle may be lowered. Also, when the open ends of the sheet bundle are opened thus, it has sometimes happened that a sheet bundle subsequently discharged enters between the open ends of the preceding sheet bundle and the sheet bundles cannot be successively reliably stacked.